A SAG ("Semi-Autogenous Grinding") mill is a cylindrically shaped grinding reduction plant of both variable diameter and length. Typically the length and diameter of such a mill have dimensions in tens of feet. The SAG mills are widely used throughout the minerals extractive industry to reduce a mineral ore of an intermediate size (approximately six inches of diameter) to less than one-eighth inch of diameter. The SAG mill accomplishes this reduction through a combination of direct impact fracturing and rotational grinding attrition. This process of reduction begins by the mineral ore being introduced into the SAG mill through a feed chute or hopper. The SAG mill is normally rotating at a relatively low rate of turn (revolutions per minute) as the ore is fed. Hardened steel balls are added to the ore feed and coupled with available larger ore particles so as to provide both direct fracturing and ore grinding attrition. The mill is designed to retain the ore until a desired product size is achieved. At this point, the ground ore is discharged through an internal port, and exits the mill for further benefication.
The nature of the intense fracturing and grinding that occurs in the SAG mill requires that the mill shell be protected to prevent deterioration. This is accomplished by lining the mill with bolted plate liners of variable size (typically five feet long by one and a half feet wide and several inches thick). These plate liners are typically made of hardened steel to effect lengthened wear rates (liner life). Further, the liners are axially cast with a raised center (lift bar) of up to four inches in length (five feet long and eight inches wide). The weight of a typical liner plate can approach 2200 pounds. There can be many tens to well over 100 liner plates installed within a SAG mill.
As the mill liner plates wear, replacement becomes necessary. This is done to protect the interior mill shell liner and to improve the efficiency of the mill by increased material lift via the new liner lift bars.
Replacement of the liner plates requires that the mill be taken out of service, the nut ends on the threaded one inch liner bolts be removed, and the liner bolts punched inwardly toward the mill. The worn liners are sequentially removed and new liners placed and bolted.
Difficulties, in the past, have occurred in trying to punch the mill liner bolts inwardly toward the mill interior. These difficulties are due to two circumstances unique to the design of the mill plate liners and attaching liner bolts. First, the liner plate typically is attached with three bolts. Each bolt can measure one inch in diameter by eighteen inches in length. The liner bolt head is a flattened "Y-end" (eared) bolt designed to set into a tapered ellipsoid hole in the liner plate. As the liner bolt is nutted and tensioned from the exterior mill shell, the bolt head seats and tensions into the tapered ellipsoid hole. As the liner wears, debris (e.g. steel ball chips, rock chips, etc.) sets into the ellipsoid bolt hole. Further, the bolt hole tends to peen over so as to develop inwardly-directed edges. This results in a "locking" of the debris within the hole and prevents easy removal of the liner bolt past the peened bolt hole.
Prior practice has been to enlarge the bolt hole during the liner plate replacement process via acetylene torch or air-arcing. Once the hole is enlarged, the bolt is pounded inwardly through the use of a sledge hammer. Unfortunately, this practice has had three major drawbacks. First, it is extremely labor intensive to physically punch the liner bolt through the liner plates. It is also very time consuming to physically enlarge the peened liner plate holes. Secondly, the lengthy process of the "sledge hammer" removal of the bolt causes up to fifty hours of down time of the SAG mill. This long period of down time can be very costly to the processing of ore and can severely interrupt the process of mineral extraction. Thirdly, the use of cutting torches within the mill presents a ventilation concern and a fume exposure problem to laborers.
It is an object of the present invention to provide a device and method for the removal of liner bolts that reduces he amount of labor required for the removal of such bolts.
It is an object of the present invention to provide a device for the removal of liner bolts that reduces the time required to remove the liner bolts from the SAG mill.
It is another object of the present invention to provide a device and method for the removal of liner bolts that creates no ventilation or fume concerns or requires no liner plate hole enlargement.
It is still another object of the present invention to provide a device for the removal of liner bolts that is relatively easy to manufacture, easy to use, and relatively inexpensive.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.